A. Field of the Invention
The present invention relates to a shadow mask for a color cathode ray tube (CRT).
B. Discussion of Related Art
A typical color CRT is illustrated in FIG. 1. On the inner surface of a panel 1 is coated with fluorescent coatings 4 and joined, by applying glass fused at about 450.degree. C., to a funnel 2 whose inner surface is coated with conductive graphite. In a mark section 7 of the funnel 2 is positioned electron guns 3 that generate and direct three separate electron beams 10. A frame 6 supports a shadow mask 5 functioning as three electrodes to perform the sorting out of the three color beams in the panel 1, and a deflection yoke 8 is placed around the neck of the funnel 2 to produce a magnetic field for deflecting the electron beams 10.
When image signals are fed into the color CRT as described above, the cathodes of the electron guns 3 emit the thermal electrons, which are accelerated and converged toward the panel 1 by voltage applied to the electrodes of the electron guns 3.
Each electron is directed by the magnetic field of a magnet 9 settled in the mask part of the panel 1 and brought to focus on the front surface of the panel 1 by the deflection yoke 8. The sorting out of the electron beams 10 is performed by the shadow mask 5 combined to the internal frame of the panel 1. The electrons from the three guns 3 pass through each slot 11 of the shadow mask 5, and thus sorted electron beams strike the respective dots on the fluorescent coatings 4 on the inner surface of the panel 1.
As the electron beams projected from the electron guns 3 are deflected by the deflection yoke 8 and pass through the slots of the shadow mask 5 to make the fluorescent coatings 4 emit lights, the shadow mask 5 permits the fluorescent coatings 4 to produce each of red, green and blue lights from the three electron beams 10 that are controlled by the three color signals.
Only about 20% of electrons can pass through the slots of the shadow mask in a prior art color CRT, and the rest of them hit the shadow mask, causing a "doming effect" by which the shadow mask is heated and swelled up. The doming effect causes a mislanding effect where the electron beams cannot impinge on the intended color dots of the screen, and by this way produces a color blotting and a deterioration in luminance and chrominance. The cause of this mislanding effect may be found in the fact that the frame and spring, functioning as a support of the shadow mask and its peripherals, are made of a metallic material that has a high thermal expansion coefficient, and particularly that the shadow mask of pure iron has such a high expansion coefficient to produce a rapid thermal deformation.